Flake morphology fine metal powders have been used for many decades by the coatings industry. They have been produced in tumbling or rotary ball mills using an organic compound to prevent cold welding of the particles and to reduce oxidation of the powder. A new potential application for flake copper powders, in particular, is electronic circuit boards produced by the thick film process. Thick-film microcircuits are fabricated from specially formulated conductive pastes (sometimes called "inks"). Pastes are commonly comprised of the following three components: (1) the functional element or elements which can be a metal, such as copper, alloy, oxide, or ceramic compound, (2) the binder which serves to bind the particles of the functional element together and to bind the element to the substrate, and (3) the vehicle which can be organic solvents and plasticizers. The vehicle allows for smooth flow in application to the substrate. In this thick-film application the powders must be finer than those previously produced.
The justification for using flake morphology powders in a copper thick film application is as follows: to reduce costs to effectively compete with air fired precious metal systems, it would be desirable to reduce the circuit trace thickness, and thus the amount of copper metal used per unit length of circuit trace. However, the trace thickness must be at least five times the particle thickness to ensure adequate electrical conductivity in the finished circuit trace. Flake morphology particles in a coating system tend to lie with their largest dimensions in the plane of the circuit board and with their thinnest dimension (thickness) in the direction of the circuit trace thickness. This allows the trace to be made thinner, and with a lower oxygen content than by using spherical particles with a diameter equal to the flake thickness (flakes have a lower specific surface area than spherical particles with a diameter equal to the flake thickness). Also the flake morphology particles are less difficult to produce than the previously mentioned spheres. The flake morphology particles must have a relatively small larger dimension as well to allow narrow line widths to be fabricated smoothly.
An upper limit of about 7-10 microns is acceptable for the flake diameter. Flake powders in this size range can be produced by conventional ball milling alone, but the resulting powders are too thin and large diameter particles are present which are difficult to classify out. In addition, milling times are excessively long to produce powders this fine.